Don't activate stars/BHs comms for subpairs that don't need it
When running over MPI, we activate the stars and BHs send/recv for any active pair or sub-pair. But, there can be many instances where we have no work activated at all in the sub-pair as no interactions between touching sub-cells will take place.
In this, MR, we change the sub-task activation to report whether anything was activated. If nothing was activated, then we also skip the comms.